The present invention relates to submarine power cables and in particular to armored submarine cables.
Ordinary cables have copper conductors, lead sheath and steel wire armor. The high density of the metals will create very high tension in the cable during laying. Such high tension may be harmful to the cable, especially to the insulation when the cable is bent over the capstan and the laying wheel of the cable ship. It is therefore desirable to use a light weight cable at parts of the route with great sea depths. This can be obtained by replacing the said metals with lighter materials, for instance copper with aluminum, lead with aluminum and steel with synthetic fibres.
The cables may also be exposed to torsional forces which could create twist of the cable. The undesirable torsional forces are usually eliminated by using a double armor or a so-called cross armor, where two armor layers are applied with opposite stranding direction.
During operation, the cable may be exposed to strain due to fishing tackle, ships anchors, etc. and in very shallow waters to wear and tear due to movement because of waves and/or sea currents. All these kinds of mechanical impacts may damage the cable.
It is possible to minimize the risk of damage to the cable in areas where such mechanical impacts may occur, for instance by embedding the cable at the sea bottom. However, such embedding may be very expensive depending on the bottom conditions, sea depth, etc. A more economical solution will therefore usually be to make the cable selfprotective by designing it with a heavy armor, as mentioned. Such a cable will withstand the mostly used fishing tackles, anchors from fishing boats etc. Experience has demonstrated that a cross-armored cable is very good with regard to such impacts. In other areas of the route, where no fishing takes place and where the risk of damage by anchoring etc. is small, the armor may then be reduced, making the cable at such parts lighter and less expensive.
Since the steel wire armor often contributes to about 50% or even more of the weight of the cable, the replacement with synthetic fibres, e.g. Kevlar, will be a very important factor in the attempt to reduce the weight of the cable. Synthetic fibres will, however, not provide sufficient protection against fishing tackles, etc. during service, but at very great sea depths fishing and anchoring are hardly of any concern at all.
The technique of using a combination of heavy armor of the cable in shallow waters and a light armor (light weight cable) where the cable passes deep waters, has been used in connection with telecommunication cables.
From Electrical Communication, Vol. 49, No. 2, 1974, pgs 177-182 there are known deep ocean type cables having non-metallic armor. There are described coaxially designed power and communication cables suitable for connecting a surface vessel with seabed installations. In order that the cable should be able to carry its own weight down to an operating depth of 6000 meters the usual steel armor is exchanged with a synthetic fiber armor essentially containing DuPont PRD-49 fibers (Kevlar).
In Electrical Communication Vol. 49, No. 2, 1974 pgs 362-369 is described the Cantat-2 Submarine Cable System which was installed between the UK and Canada in 1973. The cable route passed through shallow as well as deep waters, and the coaxial telecommunication cable was designed to have two layers of heavy gauge mild steel wire armor at depths less than 73 meters, single layer steel wire armor at depths between 73 meters and 926 meters while the deep water cable section had no protective armor. The transition from the armored cable to the light weight deep sea cable was constituted by a one nautical mile (1.85 km) long cable having a light weight armor consisting of PVC covered mild steel wires.
Inasmuch as the cable in Electrical Communication is of the communication type including a plurality of repeaters and equalizers, the joints between the various sections of the cable were made at the repeaters. The external cable armor of the armored cable sections and the center high tensile steel strength member of the light weight cable sections were probably secured to the repeater housings.